A known vacuum pumping arrangement for evacuating a chamber comprises a molecular pump which may include: molecular drag pumping means; or turbomolecular pumping means; or both molecular drag pumping means and turbomolecular pumping means. If both pumping means are included the turbomolecular pumping means are connected in series with the molecular drag pumping means. The pumping arrangement is capable of evacuating the chamber to very low pressures in the region of 1×10−6 mbar. The compression ratio achieved by the molecular pump is not sufficient to achieve such low pressures whilst at the same time exhausting to atmosphere and therefore a backing pump is provided to reduce pressure at the exhaust of the molecular pump and hence permit very low pressures to be achieved at the inlet thereof.
The turbomolecular pumping means of a molecular pump comprises a circumferential array of angled blades supported at a generally cylindrical rotor body. During normal operation the rotor is rotated between 20,000 and 200,000 revolutions per minute during which time the rotor blades collide with molecules in a gas urging them towards the pump outlet. Normal operation occurs therefore at molecular flow conditions at pressures of less than about 0.01 mbar. As it will be appreciated, the turbomolecular pumping means does not work effectively at high pressures, at which the angled rotor blades cause undesirable windage, or resistance to rotation of the rotor. This problem is particularly acute at start up conditions close to or at atmospheric pressure, when it is difficult if not impossible to rotate the rotor of the turbomolecular pumping means at high speed. Therefore, it is desirable to evacuate the turbomolecular pumping means to relatively low pressures by operating the backing pump before starting rotation of the molecular pump. An alternative but undesirable solution to the problem of turbo stage start-up, would be the provision of a much more powerful motor for driving the rotor, that would be able to overcome the windage caused by the angled rotors blades at atmospheric pressure. This solution is undesirable because, generally, a molecular pump, especially when used in the semiconductor processing industry, is kept running most of the time, and is shut down only during power failures, for servicing etc. Accordingly, a powerful motor would be needed only for a relatively small amount of the pump's operating time and therefore the increased cost of such a motor cannot be justified.
Hereto, a molecular pump and a backing pump thereof are separate units of the same vacuum pumping arrangement, the pumps being associated with respective drive shafts which are driven by respective motors. As described above, it is desirable initially to operate the backing pump to evacuate the molecular pump, prior to start-up of the molecular pump. Clearly, this would be possible only if the two pumps can be driven separately.
It is desirable to provide an improved vacuum pumping system and method of operating a vacuum pumping arrangement.